In Haaf, U.S. Pat. No. 4,167,507, the effectiveness of hydrogenated A-B-A block copolymers of vinyl aromatics (A) and dienes (B) as impact improvers for polyphenylene-ether-based compositions has been clearly demonstrated. In the patent, reference was made to certain commercial grades of hydrogenated A-B-A block copolymers, identified as Shell's Kraton GXT-0650 (now known also as G-1650), GXT-0772 and GXT-0782. These copolymers are also described in Jones, U.S. Pat. No. 3,431,323. Both patents are incorporated herein by reference.
The compositions of hydrogenated A-B-A block copolymers with polyphenylene ethers may be melt processed, e.g., by injection molding, extrusion and the like, to produce plastic articles having good physical properties. Especially noteworthy are superior resistance to impact and thermal aging, as well as excellent gloss and moldability characteristics.
The blends, with the commercial copolymers described in the '507 patent, however, tend characteristically to exhibit delamination. "Delamination" in this sense refers to the existence of poorly-adherent strata within the body of a plastic article. These layers, oriented into planes parallel to the direction of melt processing, are roughly analogous in appearance to a deck of cards viewed in cross-section. The presence of lamination can be made readily visible by cutting into a specimen molded from the composition of polyphenylene ether and A-B-A block copolymer, then twisting it. Unfortunately, under certain melt-processing conditions, this internal lamination can also become visually obvious on the surfaces of unbroken parts, becoming a defect which renders such parts undesirable on aesthetic grounds.
It has now been discovered that the aforementioned desirable properties can be obtained but without undesirable lamination, if a particular type of hydrogenated A-B-A block copolymer is used in place of the embodiments disclosed in the '507 patent. The new discovery requires the use of a high molecular weight polymer, higher than the 74,000 maximum total molecular weight in the previous embodiments, and more specifically the molecular weight of the vinyl aromatic blocks, e.g., polystyrene, should be higher than previously described in the actual embodiments of the '507 patent. Each of these vinyl aromatic blocks should be at least 15,000 and preferably from 20,000 to 40,000 in molecular weight. The diene blocks, before saturation should be greater than 50,000, making the minimum total molecular weight of the A-B-A block copolymer at least 80,000, substantially above the 74,000 maximum for the prior art embodiments.